Sahih Bukhari, Volume 4, Book54, Number 509

Narrated Abu Huraira:
The Prophet said, “Yawning is from Satan and if anyone of you yawns, he should check his yawning as much as possible, for if anyone of you (during the act of yawning) should say: ‘Ha’, Satan will laugh at him.”
Sahih Bukhari, Volume 4, Book54, Number 509